Can a non-Jewish female, who had a Jewish father and who has wholeheartedly embraced Judaism at the age of 50, legally become a prominent member of importance in a national and international Jewish women's organisation? Also can this person be a member of an Orthodox synagogue?

The term of art used here is “legally.” From the perspective of Jewish law – the relevant “legality” – there is no obligation to join or not join any organization, so that part of the question is a non-starter. Certainly, any organization has the right to determine criteria for membership, and if a Jewish organization required “Jewishness” as a prerequisite, such would be appropriate and logical. But as many “Jewish” organizations do not necessarily engage in practices or advocacy that are “Jewish” (not everyone organization of Jews is ipso facto a “Jewish organization), they could just as easily decide to have non-Jewish members as well.
The same would apply to membership in an Orthodox synagogue. Most (if not all) Orthodox synagogues require that members be Jewish. Membership conveys certain rights and privileges that would not pertain to non-Jews, and therefore different tiers of membership would be most unwieldy and unwelcome. In theory, I suppose, a particular synagogue could allow a non-Jew to be a member, but most likely only in unique cases (a longtime resident non-Jew who is pursuing conversion). Nonetheless, even in that situation, most Orthodox synagogues would withhold membership until the conversion is completed.
The broader question relates to the status of a person of any age who has a Jewish father but not a Jewish mother, or otherwise “feels Jewish.” In the context of this question, the person “wholeheartedly embraced Judaism…” Of course, as is well known according to traditional Jewish law, Jewish identity is transmitted through the mother, not the father. Why that is so is not as important as the fact that it is so, and such was universally accepted across the Jewish world until roughly 30 years ago.
Every group, community or nation establishes criteria for admission to that group, community or nation. If we analogize to the United States, the tradition of matrilineal descent was the established norm for Jewishness, much like the citizenship laws in the US that require foreigners to arrive legally, establish residency, and pass a variety of naturalization exams that, parenthetically, most indigenous Americans could not pass. But such is the law, which, granted, is in flux today. No group of Americans, however well-meaning, has the right to establish its own criteria for American citizenship in defiance of the federal standards. The same pertains to Jews. No group of Jews has the right to unilaterally change the criteria for Jewishness as ordained in the Talmud and Codes. (Some citizenship codes are quite restrictive. E.g., a person who wants to become a citizen of Liechtenstein must pass seven rounds of screening culminating in the personal approval of the President. It is not lightly undertaken and is not left to feelings.)
One might just as well ask: what if the child of two non-Jews “wholeheartedly embraced Judaism”? Would the embrace alone transform that person into a Jew? Certainly not. Jewish identity is not only a question of personal commitment but also of membership in a national entity. As such, the “nation” has to approve membership, which is done through adherence to the halachic norms and the authorized mechanisms of conversion. Bear in mind that we are not only a religion but also a nation – we are a religio-nation, in what was a new phenomenon in the world. Thus, the child of a Jewish father only remains a non-Jew in the eyes of Jewish law.
Undoubtedly, the child of a Jewish father has the advantage – if relevant to the case – of already having an ethnic attachment to the Jewish people. That often helps the person feel more connected to the Jewish people and, in cases of true conversions (of which I have presided over many), eases the integration into full Jewish life. But can a person “wholeheartedly embrace Judaism” and reject its norms, laws, customs and criteria for membership? Notwithstanding her emotional commitment, I do not see how such is possible. The “embrace,” by definition, is far less than “wholehearted,” and it would behoove this person to join the Jewish people under the traditional guidelines.

At the beginning of the twenty-first century, Jewish identity remains a complex matter. It is an ideological and halachic issue that often divides Jews from one another. The question you are asking, then, cannot be answered with absolute certainty because religious ‘movements’ and even Jewish organizations have different policies with regard to membership and affiliation.

Traditionally, Jewish identity is defined by one’s mother. That is, if one has a Jewish mother, then one is considered Jewish according to Conservative and Orthodox rabbis; if one’s father is Jewish but ones’ mother is not, then one would not be considered Jewish without undergoing a formal ceremony conversion, ‘according to Jewish law.’ This would involve a visit to the mikveh, a ritual pool for both men and women, and either circumcision or a ceremony called hatafat dam brit, for men.

The Reform and Reconstructionist movements embraced a policy in favor of patrilineal decent so that any person who has a Jewish parent and identifies himself or herself as Jewish would be considered Jewish.

As a Conservative Rabbi, I often encounter people who have non-Jewish mothers and Jewish fathers. While conversion is necessary in such cases, I usually treat such conversions differently than I would a person who seeks to convert to Judaism without any prior Jewish lineage. This is particularly the case when the person in question has been raised as a Jew without a formal conversion. In such cases, I explain that they while they are Jewish in identity, they need to formalize their identity and practice in law, much as a person born in a foreign country but raised in the United States may feel American but not have the benefit of citizenship because they were not formally naturalized.

Jewish organizations may vary in terms of their policy on Jewish identity. National and international organizations that are synagogue based may have the same requirements as the synagogue. Many other organizations, because they have a non-religious in nature, do not question the basis of one’s Jewish identity. Simple identification is enough. This would not be the case for an Orthodox or a Conservative synagogue. In such cases, a conversation with the rabbi of the congregation will help clarify what is expected of members.

Following the ritual of conversion, you would be considered Jewish and there should be no barriers to your full participation in the synagogue and its various organizations. In fact, the Talmud prohibits reminding a person of their prior status. The person who undergoes conversion is considered no different than someone who is born to two Jewish parents.

Your acceptance or non-acceptance as a Jew by an organization or a synagogue is not a judgment of you as a person. It is recognition that we live in a complex world where we don’t all agree on what it is that defines Jewish identity. Your passionate sense of Jewish identity should be honored and respected but at the same time, I hope you will be respectful of the need of people to define Jewish identity based on their understanding of Jewish tradition and their conscience.

The answer to the question will depend on the by-laws and rules and regulations of the organization and Synagogue in question. As a point of information, I know of several Reform Synagogues that allow non-Jews to be members but this must be answered on a case by case basis.

THE VIEWS EXPRESSED IN ANSWERS PROVIDED HEREIN ARE THOSE OF THE INDIVIDUAL JVO PANEL MEMBERS, AND DO NOT
NECESSARILY REFLECT OR REPRESENT THE VIEWS OF THE ORTHODOX, CONSERVATIVE OR REFORM MOVEMENTS, RESPECTIVELY.